Mechanical joints and hand-manipulated mechanical joints find uses across multiple industries and consumer applications.
Our first introduction to mechanical joints usually comes in children's toys. Christiansen's patent (3,005,282), the Lego® mechanical joint, for example, is a friction-based joint. To connect the joint, the user overcomes friction while sliding the pieces together. That same friction holds the pieces together. While tiny, the Lego® mechanical joint suffers from one primary drawback: low strength. In tension, the separation force is roughly equal to the connecting force. Increasing the joint strength makes connecting more difficult.
The cotter pin, similar to Smith's patent (362,548), on the other hand, forms a strong joint under any direction of imposed force. Under compression, tension, shear, and torsion, a cotter pin joint remains together. Additionally, if more holes are drilled for the pin, the joint can be rotated and re-connected in set rotational increments. Yet, the cotter pin suffers two disadvantages. Firstly, because of alignment and dexterity, connecting and separating can be difficult, particularly when the connecting structures are smaller than a finger. Secondly, because of an over-sized hole for the pin, wiggle remains after the joint has been formed, losing some dimensional accuracy.
The buckle, similar to Tracy's patent (4,150,464), finds uses in many consumer applications. It remains strong under any direction of imposed force, is easy to connect and separate, and is fairly dimensionally accurate. However, the buckle suffers some disadvantages. The joint strength is less than a cotter pin, because the male-side must be composed of materials flexible enough to snap-in. The buckle can only be connected in 180° rotational increments. Finally, the buckle is large for hand-manipulated joints, because fingers must be able to press the engaging flexible clasping arms.